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JsonCodec

Interfaces to serialize and deserialize PHP objects to/from JSON.

Additional documentation about this library can be found on mediawiki.org.

Usage

To make an object serializable/deserializable to/from JSON, the simplest way is to use the JsonCodecableTrait and implement two methods in your class, toJsonArray() and the static method newFromJsonArray():

A slightly more complicated version of this example can be found in tests/SampleObject.php.

If your class requires explicit management -- for example, object instances need to be created using a factory service, you can implement JsonCodecable directly:

A full example can be found in tests/ManagedObject.php.

Note that array returned by toJsonArray() can include other JsonCodecable objects, which will be recursively serialized. When newFromJsonArray is called during deserialization, all of these recursively included objects will already have been deserialized back into objects.

To serialize an object to JSON, use JsonCodec:

In some cases you want to embed this output into another context, or to pretty-print the output using non-default json_encode options. In these cases it can be useful to have access to methods which return or accept the array form of the encoding, just before json encoding/decoding:

Handling "non-codecable" objects

In some cases you want to be able to serialize/deserialize third-party objects which don't implement JsonCodecable. This can be done using the JsonCodec method ::addCodecFor() which allows the creator of the JsonCodec instance to specify a JsonClassCodec to use for an arbitrary class name. For example:

This is done by default to provide a serializer for stdClass objects.

If adding class codecs one-by-one is not sufficient, for example if you wish to add support for all objects implementing some alternate serialization interface, you can subclass JsonCodec and override the protected JsonCodec::codecFor() method to return an appropriate codec. Your code should look like this:

A full example can be found in tests/AlternateCodec.php.

Enumerations

PHP 8.1 enum types are codecable by default. BackedEnum types serialize as [ 'value' => ... ] using their backing value, and other enum types serialize as [ 'name' => ... ] using their case name. You can customize the serialization of the enum either by having your Enum type implement JsonCodecable, or by using the ::addCodecFor() mechanism used for "non-codecable" objects. Either of these is used in preference to the default JsonEnumClassCodec which handles enums.

More concise output

By default JsonCodec embeds the class name of the appropriate object type into the JSON output to enable reliable deserialization. In some applications, however, concise JSON output is desired. By providing an optional "class hint" to the top-level call to ::toJsonArray() and newFromJsonArray() and implementing the ::jsonClassHintFor() method in your class codec you can suppress unnecessary type information in the JSON when your provided hint matches what would have been added. For example:

You can then generate concise output by providing the proper hints when serializing and deserializing:

Note that the provided value is a hint. If we were to put a value other than a SampleObject into the SampleContainerObject the type of that value would be embedded into the JSON output, but it would not break serialization/deserialization.

As illustrated with the foos property, to indicate a homogenous list or array of the given type, you can pass Hint::build(...., HintType::LIST) as the class hint. A stdClass object where properties are values of the given type can be hinted with Hint::build(...., HintType::STDCLASS).

A full example can be found in tests/SampleContainerObject.php.

The HintType::USE_SQUARE modifier allows ::toJsonArray() to return a list (see array_is_list) and have that list encoded as a JSON array, with square [] brackets.

The HintType::ALLOW_OBJECT modifier ensures that empty objects are serialized as {}. It has the side effect that ::toJsonArray() may in some cases return an object value instead of the array value implied from the method name.

The USE_SQUARE and ALLOW_OBJECT hints are necessary because normally JsonCodec attempts to encode all object values with curly {} brackets by inserting a _type_ property in the encoded result when necessary to ensure that the encoded array is never a list. PHP's json_encode will use {} notation for non-list arrays. If you don't want the added _type_ property added to your encoded result, then you need to specify whether you prefer [] notation (USE_SQUARE) or {} notation (ALLOW_OBJECT) to be used in ambiguous cases.

An example with hint modifiers can be found in tests/SampleList.php and its associated test cases.

Where a superclass codec can be used to instantiate objects of various subclasses the HintType::INHERITED modifier can be used. An example of this can be found in tests/Pet.php, tests/Dog.php, and tests/Cat.php and their associated test cases in tests/JsonCodecTest.php.

For forward-compatibility with JSON serialized with hints, you may specify HintType::ONLY_FOR_DECODE to indicate that serialization should ignore the hint (encoding full class information) but that deserialization should take the hint into account, so that it is possible to read encodings produced either with or without a future hint.

In some cases, ::jsonClassHintFor() may be inadequate to describe the implicit typing of the JSON; for example tagged union values or implicitly-typed objects nested deeply or inside non-homogeneous arrays. For those use cases a JsonCodecInterface parameter is provided to the ::jsonClassCodec() method. This allows the serialization/deserialization code to manually encode/decode portions of its JSON array using an implicit type. More details can be found in the interface documentation for src/JsonCodecInterface.php and a full example can be found in tests/TaggedValue.php.

Further customization of the encoding of class names and class hints is available using the protected methods JsonCodec::isArrayMarked(), JsonCodec::markArray() and JsonCodec::unmarkArray(). A full example can be found in tests/ReservedKeyCodec.php.

Type abbreviations

The hint mechanism allows encodings to omit explicit type information in many cases, but generic container types may not be able to fully hint their contents. Further, for cross-platform use it is useful to have a way to explicitly encode class types without using the literal name of a PHP class. The type abbreviation mechanism allows the codec to define a standard set of abbreviations for PHP classes. Without hinting, the encoded JSON output might look like:

Calling JsonCodec::addAbbrev() allows you to define an abbreviation:

and subsequently you can encode/decode:

A cross-platform encoder can recognize @my-type-abbreviation and substitute an appropriate class implementation in a non-PHP language.

Although JsonCodec supports PHP's class_alias, abbreviations also provide another way to allow the PHP implementation class to be renamed or renamespaced without breaking decode.

For forward-compatibility, abbreviations added using HintType::ONLY_FOR_DECODE won't be used in the encoded string:

Finally, the codec abbreviation registry provides a mechanism to generically provide hints for encode/decode. For example, a JSON-valued attribute mechanism for HTML might use the attribute name as the default hint to allow a generic getter:

Abbreviations can be added to the codec using JsonCodec::addAbbrev(), and registered abbreviations can be retrieved from the codec using JsonCodec::getAbbrev() as shown above.

Running tests

History

The JsonCodec concept was first introduced in MediaWiki 1.36.0 (dbdc2a3cd33). It was split out of the MediaWiki codebase and published as an independent library during the MediaWiki 1.41 development cycle, with changes to the API.